I work at a place where they have access to very sensitive data and think a lot about security. I dont want to ask bc they may lie, is there software to monitor data such as email (pop/imap AND webmail such as hotmail or gmail), forum post etc. Basically any POST data. I am worried that if i read or wrote an embarrassing email to (girl)friend or say something that can be considered suspicious; that it would come back to bite me in the behind (either laughed at or have me put under surveillance).
Yes, there's a number of techniques that could conceivably be used by an employer to monitor computer use. Are you talking about working someplace where there's security clearances?
Does your work prohibit personal use of work computers? If so, don't do it; not even a little.
Without listing the myriad ways of doing this, yes there are applications and techniques for sniffing network traffic including email, instant messaging, web browsing, etc., etc. There isn't any way you're going to discover on your own whether or not your communication and computer activity has been monitored.
My advice would be to keep your communication and computer activity at work related to work. Save the private/personal stuff for your own computer on your own time.
Your employer can monitor you in a number of given ways ranging from monitoring the PC itself via software, screenshots, keystroke logger, etc to monitoring the traffic itself as it passes through a firewall appliance such as an IPCop box, pfSense box, Smoothwall box, etc.
In short... the computer isn't yours (therefore not to be trusted) and the internet connection isn't yours (therefore not to be trusted).
The best advice I could give you to be safe would be to tell you to ASSUME that everything you do on your computer is monitored.
Always keep this in the back of your mind and you'll be just fine.
As a side note... I've personally seen screenshots between girlfriends and boyfriends... seen screenshots of personal online banking balances... seen screenshots of people surfing porn at work... seen employees bitching about other employees or their bosses... seen employees planning on quitting, etc, etc.
The sad part is... a lot of stuff is done while the person is in a bad mood and just venting. The REALLY sad part is... they're usually over it an hour or two later... but the captures are (or can be) FOREVER.
Just things to keep in mind.